1. Field of the Invention
This invention relates to epitaxial growth of graphene, and more particularly to the engineering of defects in graphene sheets.
2. Description of the Related Art
Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb, hexagonal crystal lattice. Graphene is a basic building block for graphitic materials of all other dimensionalities. Graphene can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.
Graphene has rather remarkable properties. Graphene is stable, chemically inert, and crystalline under ambient conditions. It is a semimetal in that its conduction and valence bands just meet at discrete points in the Brillouin zone. An electron in graphene has an effective mass of zero and behaves more like a photon than a conventional massive particle. Finally graphene can carry huge current densities—about 108 A/cm2, roughly two orders of magnitude greater than copper.
Epitaxy refers to the method of depositing a monocrystalline film on a monocrystalline substrate. The deposited film is denoted as an epitaxial film or epitaxial layer. The term epitaxy comes from the Greek roots epi, meaning “above”, and taxis, meaning “in ordered manner”. It can be translated “to arrange upon”. Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. Techniques for epitaxy deposition include but are not limited to Molecular Beam Epitaxy (MBE), Chemical Vapor Deposition (CVD) and precipitation methods through annealing of implanted materials in the substrate.
Epitaxy may be used to a create single or a few sheets of graphene. As shown in FIGS. 1a and 1b a substrate 10 is provided that includes a single crystal region on the surface of the substrate. This region has a honeycomb, hexagonal crystal lattice substantially lattice-matched to graphene. Typical substrates include Silicon-Carbide (SiC) and elements from the periodic table including iron to copper, ruthenium to rhodium and rhenium to iridium and mixtures thereof. Carbon atoms 12 are deposited at the vertices of three adjacent atoms 14 in the substrate. The distance between these vertices is approximately the bond length of graphene. Considerable effort has been made to refine the epitaxial process in order to create perfect graphene.
More recently researchers have investigated defect structures in graphene and methods of processing the graphene to create such defect structures. As shown in FIG. 2, the hexagonal crystal lattice 20 of graphene may be disrupted to create, for example, Stone-Wales defect pairs 22. A “defect” may be any deviation from the perfect honeycomb, hexagonal lattice structure. Thermal annealing, mechanical strain and electron radiation of graphene have been proposed as techniques to create defect structures. Mark T. Lusk et al “Nano-Engineering Defect Structures on Graphene” Condensed Matter, Materials Science, 6 Dec. 2007 discloses a number of possible defect structures including blisters, ridges, ribbons and metacrystals. Lusk discloses that the defect structures may be synthesized by electron radiation or thermal activation of the graphene; a sheet of graphene is formed and then radiated or thermally activated to from the defect structures. Jannik C Meyer “Direct Imaging of Lattice Atoms and Topological Defects in Graphene Membranes” Nano Letters 2008 Vol. 8, No. 11 3582-358 uses an electron beam associated with an electron microscope to both create Stone-Wales defect pairs in graphene and to view the defects; a sheet of graphene is formed then radiated with an electron beam to form the defects.
Because of its unique electrical properties, researchers are also investigating the use of graphene in electronic devices such as transistors and integrated circuits. U.S. Pat. No. 7,619,257 discloses a device in which one epitaxial layer of graphene is disposed on a lattice-matched substrate. A multi-layered, single crystal, electrically insulative second region that is lattice-matched to graphene is disposed on the graphene. In an embodiment, a channel region of a field effect transistor (FET) is formed in the graphene.